Is it religious?
The current secular mindfulness programmes have been described as a coming together of contemplative traditions and modern Western science.
Although, for millennia, many different world religions have incorporated contemplative meditation into their form of practice, Buddhist psychology has most to say about the ‘how’ to meditate. Some of these approaches have informed the development of mindfulness programmes.
However, it’s really important to know that secular mindfulness programmes such as MBSR are concerned with understanding our thinking mind and developing ways to avoid being hijacked by it in ways that don't help us.
MBSR and other Mindfulness-based programmes are non-religious and benefit those of all religions and none.